change all entity Class fields into String fields

Description

I saw that http://jira.codehaus.org/browse/GRAILS-6020 was rejected as won't fix. This means that we cannot use name(unique:entity?) in TemplateField? in gdt, which means that we have no easy way of enforcing unique template names per entity in the data model. The user can potentially be confused by allowing different templates with the same name.

The only way out I see here is to change our definition of 'Class entity' in Template and TemplateField? into 'String entity'. All code that uses the entity (e.g. in the Bootstrap) should then be changed accordingly. E.g. in BootstrapTemplates?: